Precambrian Research 105 (2001) 357 – 369
Precambrian basement character of Yemen and correlations
with Saudi Arabia and Somalia
Martin J. Whitehouse
a,*, Brian F. Windley
b, Douglas B. Stoeser
c,
Salah Al-Khirbash
d, Mahfood A.O. Ba-Bttat
e, Abdullah Haider
faSwedish Museum of Natural History,Box50007,SE-104 05Stockholm,Sweden bDepartment of Geology,Uni6ersity of Leicester,Leicester LE1 7RH,UK cUnited States Geological Sur6ey,MS905,Den6er Federal Center,Den6er,CO80225,USA
dUni6ersity of Sana’a,P.O.Box13499,Sana’a,Yemen
eDepartment of Earth and En6ironemental Sciences,Kuwait Uni6ersity,P.O.Box5960,Safat,13060,Kuwait fGeological Museum,Øster6oldgade5-7,1350Copenhagen K.,Denmark
Received 23 February 1999; accepted 27 May 1999
Abstract
The Precambrian basement of Yemen occupies a key location in the Pan-African orogen of Gondwana. This paper reviews geological, isotopic and geochronological data and presents new Pb- and Nd-isotope data which help define distinct gneiss terranes within this basement, constraining correlations of these terranes with neighbouring regions of Saudi Arabia and Somalia. Existing whole-rock Pb- and Nd-isotopic data are also summarised. These data should facilitate a more objective assessment of the contribution of the Yemen Precambrian to Cenozoic magmatism associated with the opening of the Red Sea and the Gulf of Aden. © 2001 Elsevier Science B.V. All rights reserved.
Keywords:Pb- and Nd-isotopes; Precambrian; Yemen
www.elsevier.com/locate/precamres
1. Introduction
The Precambrian basement of Yemen is located between the collage of low-grade, mainly island arc terranes of the Arabian – Nubian Shield (ANS) to the west and the high-grade polycyclic, mainly gneissic terranes of the Mozambique Belt to the
south, which extends via Somalia and Ethiopia to eastern Africa. Thus, the Precambrian basement of Yemen provides a key link in our understand-ing of the Pan-African orogen of Gondwana (Kro¨ner, 1985; Stern, 1994). The Precambrian rocks of Yemen underlie much of the Cenozoic volcanic cover and therefore the trends, linea-ments, varied age and heterogeneous character of the Precambrian rocks may also have had a sig-nificant structural and chemical influence on the evolution of the volcanic-dominated continental margins of Yemen.
* Corresponding author. Fax: +46-8-5195-4031.
E-mail address: martin.whitehouse@nrm.se (M.J. White-house).
M.J.Whitehouse et al./Precambrian Research105 (2001) 357 – 369 358
A number of recent studies have been published on the Precambrian geology of eastern Saudi Ara-bia (Stoeser and Stacey, 1988; Quick, 1991; Agar et al., 1992) and the Precambrian rocks of north-ern Somalia (Sacchi and Zanferrari, 1987; Lenoir et al., 1994; Kro¨ner and Sassi, 1996). Added to this, there is an expanding literature on the Pre-cambrian basement of Yemen (Stoeser et al., 1991; Whitehouse et al., 1993, 1994, 1998; Wind-ley et al., 1996), which is likely to have consider-able influence upon the development of Cenozoic volcanism and structure in the region. In their geochronological study of Yemen basement rocks, Whitehouse et al. (1998) proposed tentative corre-lations with Precambrian basement rocks of Saudi Arabia and Somalia. The aim of the present paper is to review the main tectonic units and isotopic characteristics (existing geochronological and Nd-isotope data; new Pb- and Nd-Nd-isotope data) of the Yemen basement and further outline correlations with Saudi Arabia and Somalia.
2. Yemen basement
2.1. Geological obser6ations
The lithological associations of the terranes in Yemen are summarised in Table 1 (modified after Whitehouse et al., 1998). In this paper, we use the term terrane to imply a tectonic unit characterised by distinctive geological, geochemical, isotopic and/or geochronological features within the boundaries of major structural discontinuities. From a combination of the geological relation-ships and our geochronological data, we conclude that the Precambrian basement of Yemen is com-prised of an alternation of early Precambrian gneissic terranes and late Proterozoic island arcs, which were accreted together to form an arc-gneiss collage during, the Pan-African orogeny.
This accretion gave rise to NNE/NE-trending terranes with prominent parallel lineaments which are principally the suture zones and associated mylonite zones along and close to the boundaries of the alternating terranes. For example, a major suture possibly correlative with the Nabitah su-ture of Saudi Arabia (Fig. 1) is manifested near
Hajjah by a \30 m wide, vertical, highly de-formed ophiolite consisting of alternating tectonic slices of serpentinites and gabbros. This is the southern end of a 1200 km long suture zone (Quick, 1991), which is a major zone of weakness and fluid transport in the continental crust. Fur-ther southeast the suture zones between the Abas, Al-Bayda, Al-Mahfid and Al-Mukalla terranes are less prominent than the Nabitah suture but nevertheless are represented today by zones of ductile and brittle deformation several kilometres wide. This is seen along the suture between the Abas and Al-Bayda terranes where there is also a 1 km wide mylonite zone. The suture zones be-tween the gneiss and arc terranes in Yemen are major crustal scale tectonic boundaries, which might be expected to have influenced the develop-ment of later geological structures such as sedi-mentary basins and lava fields.
2.2. Isotopic and geochronological features
2.2.1. Nd-isotopic data
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Table 1
Comparative geology and isotopic data from terranes in Yemen, Saudi Arabia and northern Somalia
Yemen/Saudi Arabia Northern Somalia
Isotopic data
Description Terrane Description
Terrane Isotopic data
(complex) Asir Saudi Amphibolite-facies gneisses Yemen: none. Saudi Arabia:
alternating with 840–750 Ma diorites and Arabia &
greenschist-grade supracrustal
?Yemen tonalites; evolved granitoids
\570 Ma1 belts
Amphibolite-facies
Afif Saudi Yemen: none. Saudi Arabia:
Arabia & orthogneisses alternating with basement — 1800 Ma (UPb);tDM2400–1650 Ma; ?Yemen meta-volcanic belts. Many
750; Siham arc — 750–700 granitic plutons (Andean-type
continental margin?) Ma2
Abas Yemen Amphibolite-facies Orthogneisses — tDM 2300–1300 Ma3,4; 950–750 orthogneisses containing belts
Ma UPb5
of supracrustal rocks (rhyolites, schists,
amphibolites, tuffs, marbles)
Al Bayda Greenschist-grade island Granitoids — tDM Abdulkadir Greenschist-grade basalts and Basalts: 700–640 Ma7 Yemen arc-type rhyolites, andesites, 2500–2000 Ma4; Dykes — tuffs, phyllites, quartzites,
marbles 700–600 Ma ArAr6
basalts and tuffs. Granitic plutons. Ophiolites. Andesite-rhyolite dykes
Al-Mahfid Amphibolite-facies Old gneisses — tDM Mora & Qabri Amphibolite facies gneisses Granitic gneisses: 840 –720 Yemen orthogneisses containing 3000–2700 Ma4; 2900–2550 Bahar containing amphibolites, Ma UPb; xenocrysts:
gabbro bodies and two Ma UPb5; granitoids — marbles, quartzites, and 1820–1400 Ma8
generations of supracrustal tDM2200–1300 Ma4; 750 Ma gabbro bodies. Granulite relics.
UPb5
rocks (amphibolites, quartzites, marbles, rhyolites)
None Maydh (Mait) Greenschist-grade basalts,
Greenschist-grade island Basalts: 700-640 Ma7
Al-Mukalla
tuffs, pelitic-clastic sediments arc-type tuffis, rhyolites,
Yemen
basalts, lava breccias. Granitic plutons, mafic dykes
Granite — 562 Ma9
Ghabar group Very low-grade clastic Inda Ad group Very low-grade clastic Granites: 550 Ma7
sediments and limestones. sediments and limestones.
Yemen
Granites. Late Granites. Late
Proterozoic–Cambrian Proterozoic-Cambrian
1Quick (1991) summarising data of Calvez et al. (1983) and Stoeser (1985). 2Agar et al. (1992).
M
Pb- and Nd-isotopic data from the Precambrian of Yemena
207Pb/204Pb
Y92-5/1 44°59%E 14°22%N 17.465 15.543
38.687 mig-gn 45°1.7%E 14°21.0%N 18.198 15.661
Y92-7/1
Y92-7/3 mig-gn ‘‘ 18.137 15.649 38.724
38.814 18.445 15.678
Y92-7/4 mig-gn ‘‘
38.637
mig-gn ‘‘ 18.029 15.660
Y92-7/5
Y92-7/6 mig-gn ‘‘ 18.337 15.670 39.206
38.871 mig-gn
Y92-7/7 ‘‘ 18.313 15.665
38.825
mig-gn ‘‘ 18.026 15.673
Y92-7/8
40.151 granite 45°1.7%E 14°20%N 18.027 15.676
Y92-9/1
41.046 gr-gn
Y92-12A/1 45°1.7%E 14°16%N 18.188 15.632
40.836
gr-gn ‘‘ 18.227 15.668
Y92-12A/3
Y92-12A/6 gr-gn ‘‘ 18.068 15.772 40.530
39.711 gr-gn
Y92-12B/1 ‘‘ 18.051 15.734
40.197
gr-gn ‘‘ 18.093 15.775
Y92-12B/2
40.016
gr-gn ‘‘ 17.979 15.612
Y92-12B/3
39.167 18.001 15.745
Y92-12B/4 gr-gn ‘‘
39.879
gr-gn ‘‘ 18.031 15.700
Y92-12B/5
40.483
gr-gn ‘‘ 18.447 15.741
Y92-12B/6
39.095 Y92-15/1 gr-gn 45°20.7%E 14°17.7%N 17.930 15.690
Y92-15/2 gr-gn ‘‘ 17.761 15.702 39.149
38.709 17.802 16.202
Y92-15/5 aug-gn ‘‘
38.331
gr-gn 45°05.1%E 14°21.1%N 17.621 6.014 31.68 0.1147 0.511793 (20) 2.01 15
.614 MJG76-19B
4.087 21.77 0.1135 0.512108 (8) 1.51 MJG76-24 gr-dyke 45°10.9%E 14°19.5%N
5.661 34.26 0.0999 0.511725 (11) 1.84 38.058
gr-gn
MJG76-25 45°13.1%E 14°18.1%N .414 15.606
37.842
granite 45°21.8%E 14°15.2%N 17.485 15.99 73.08 0.1323 0.51263 (5) 1.38 15
.561 MJG76-29A
7.582 37.70 0.1216 0.512348 (7) 1.25 MJG76-32A granite 45°23.7%E 14°10.5%N 17.596
15
.552 37
.756
3.710 19.97 0.1182 0.512107 (10) 1.59 MJG76-10 metased 44°54.9%E 14°24.6%N
metased 45°10.9%E 14°19.5%N 2.227 10.67 0.1261 0.511224 (17) 3.29
MJG76-23
2.155 10.35 0.1258 0.511271 (9) 3.19 MJG76-23 rpt metased ‘‘
2.831 17.68 0.0968 0.512305 (6) 10.3 metased
MJG76-31 45°22.5%E 14°12.6%N
38.262
gneiss 44°40.0%E 15°11.4%N 17.549 15.599 0.511944 (10)
F25
15.547 37.933 0.512073 (10)
gneiss
F27 44°41.3%E 15°12.0%N 17.322
Al-Bayda island arc
45°27’E 14°07’N 17.251 15.498 gabbro
Y92-19/1 37.153
15.637 40.678 45°43’E 14°03’N
Y92-34/1 granite 18.702
15.642 38.851 45°’44’E 14°04’N
Y92-35/1 granite 18.017
40.065 Y92-37/1 granite 45°’42’E 13°57’N 18.297 15.706
Y92-39/1 granite 45°48’E 13°54’N 19.184 15.665 41.893 41.518 19.186 15.674
BY-63 dyke-host 45°47.2’E 13°53.7’N
37.870 gabbro 45°46.0’E 13°52.0’N 17.320 15.427
BY-111
37.599 diorite 45°46.4%E 13°52.0%N 17.914 15.605
BY-113
M
Table 2 (Continued)
207Pb/204Pb
Locality 206Pb/204Pb 208Pb/204Pb
Lithologya Sm ppm Nd ppm 147Sm/144Nd 143Nd/144Nd t
DM(Ga) Sample
BY-117 qtz-diorite 45°47.4%E 13°52.0%N 17.944 15.589 38.860
1.196 6.521 0.1109 0.512414 (9) 1.02 37.219
gr-dyke 45°39.7%E 13°59.5%N 16.871 .451 4.533 29.86 0.0917 0.511622 (7) 1.85 MJG76-36A
3.768 15.75 0.1446 0.511971 (8) 2.51 MJG76-42A diorite 45°45.8%E 14°03.7%N
4.042 19.61 0.1246 0.511433 (17) 2.87 37.357
.115 .498 MJG76-43 diorite 45°48.2%E 14°04.8%N
37.239
aug-gn 45°34.0%E 14°01.2%N 16.928 .453 0.843 4.237 0.1202 0.512078 (8) 1.66 MJG76-52C
gr-gn 45°32.3%E 14°02.9%N 5.036 23.53 0.1293 0.512081 (17) 1.84
MJG76-53A
2.531 13.30 0.1150 0.511783 (5) 2.04 37.695
6.487 30.89 0.1270 0.511961 (6) 2.00 MJG76-49B metased 45°32.3%E 13°57.7%N
4.762 16.66 0.1728 0.512789 (9) 1.12 metased 45°27.5%E 14°05.5%N
MJG76-57A
Al-Mahfid terrane
15.848 42.961 46°55.1%E 14°3.0%N
Y92-47/1 gr-gn 21.472
15.730 41.341 46°54.4%E 14°2.7%N
Y92-48/1 gr-gn 19.449
42.371
Y92-48/2 gr-gn ‘‘ 27.793 17.246
Y92-48/3 gr-gn ‘‘ 21.497 15.859 42.659
42.094 24.706 16.975
Y92-49/1 gr-gn 46°46%E 14°02%N
40.392 gr-gn 46°51.6%E 14°3.0%N 18.842 15.921
Y92-50/1
42.015 gr-gn 46°51.0%E 14°3.0%N 21.496 16.395
Y92-51/1
45.242 Y92-52/1 leucogn 46°45%E 14°02%N 37.599 18.404
43.239 gr-gn 46°0.6%E 13°53.7%N 18.432 15.655
BY-15E
41.501 19.317 15.705
BY-21 gr-gn 45°57.8%E 13°50.3%N
44.368 gr-gn 46°2.7%E 13°51.9%N 23.096 16.645
BY-30A
BY-30B gr-gn 46°2.5%E 13°51.5%N 28.362 17.362 39.707 38.640 gr-gn
BY-36B 45°59.7%E 13°50.1%N 19.893 16.059
38.711 gr-qn 45°47.4%E 13°42%N 17.654 15.569
BY-92
40.829 granite 45°59.4%E 13°53.1%N 19.400 15.743
BY-18A
41.149 19.459 15.748
BY-18B granite 45°59.4%E 13°53.1%N
40.452 granite 45°54.5%E 13°49.8%N 19.458 15.703
BY-19A
40.632 granite 46°2.8%E 13°51.8%N 19.026 15.745
BY-30C
43.433 BY-36C granite 45°59.7%E 13°50.1%N 18.934 15.753
39.853 granite 45°45.8%E 13°42.1%N 17.642 15-602
BY-91
39.402 granite
BY-93 45°48%E 13°42.6%N 17.689 15.579
41.365 granite 46°0.2%E 13°53.6%N 18.229 15.636
BY-106
BY-16K2 amphibolite 45°54.1%E 13°46.5%N 18.198 15.617 37.968 37.918 17.859
BY-24A amphibolite 45°55.7%E 13°48.4%N 15.624 West Yemen
F4 amphibolite 43°25.0%E 15°1.0%N 18.921 15-559 37.558 0.512860 (10)
0.513180 (10) 37.572
19.127 15-586
F6 granite ‘‘
38.378
amphibolite 43°23.3%E 14°58.0%N 19.172 15.603 0.512755 (10)
F22
16.658 39.011 0.512721 (10)
‘‘
F23 gneiss 19.927
aAbbreviations in lithology column: mig-gn, migmatitic gneiss; gr-gn, granitic gneiss; gr-dyke, granitic dyke; leucogn, leucogneiss; aug-gn, augen gneiss; metased, metasediment. Standard ion exchange separation techniques for Pb were followed. Samples were analysed on a VG 54E mass spectrometer in Oxford (Y92 series) or a Finnigan MAT262 mass spectrometer at USGS, Menlo Park (MJG76 series). Ratios in italics indicate measurements on separated feldspars. Pb-isotopic ratios are corrected for mass fractionation of−0.15% per atomic mass unit derived from replicate measurements of NBS 981 standard. Overall analytical error isc90.1% (2s). SmNd data (MJG76 series) were obtained at USGS, Menlo Park using analytical techniques described by Whitehouse et al. (1992) (Nd-isotope ratio reproducibility9
M.J.Whitehouse et al./Precambrian Research105 (2001) 357 – 369 362
African age (c760 Ma) intrusives and/or gneisses in the Al-Mahfid and Abas terranes show a range of model ages, which indicate the involvement of older crust in their genesis and, for the Abas terrane, the observation of model ages as old asc
2.3 Ga, together with the documentation of a 2.6 Ga core in a 760-Ma zircon (Whitehouse et al., 1998) suggests the possibility that this crust might be Archean. Samples from the Al Bayda arc terrane exhibit a wide range of SmNd model
ages, not only in granitoids, which might be ex-pected to sample underlying gneissic basement to the arc, but also in more mafic samples (e.g. diorite MJW76-43 which has a tDM of 2.87 Ga).
The Afif terrane data are quite different from any of the Yemen terranes for which isotopic data
exist, both in crystallisation age (since no mid-Proterozoic ages have yet been recorded from Yemen) and in depleted mantle model age charac-teristics. It should be stressed, however, that sam-pling in both regions has not, to date, been extensive.
2.2.2. Pb-isotopic data
In Table 2 and Fig. 3, we present whole-rock Pb-isotopic data from a variety of lithologies within the Abas and Al-Mahfid gneiss terranes and the Al-Bayda island arc terrane, as well as feldspar analyses for the Grolier et al. (1977) samples from the Abas and Al Bayda terranes. Additionally, we present data for four samples from western Yemen (together with two Abas
M.J.Whitehouse et al./Precambrian Research105 (2001) 357 – 369 363
Fig. 2. Histogram of depleted mantle model ages (tDM; De-Paolo et al., 1991) for granitoids, granitic gneisses and mafic rocks (M) from the Abas, Al-Bayda and Al-Mahfid terranes of Yemen. Data from this study and Windley et al. (1996). Data from pre-750 Ma basement rocks from the Afif terrane of Saudi Arabia are shown for comparison (Agar et al., 1992; P-paragneiss). Numbers for some samples refer to UPb
zir-con crystallisation ages (in Ga; Whitehouse et al., 1998).
subsequently, it remains possible to make mean-ingful first order comparisons for the uranogenic Pb system (235U and 238U decaying, respectively,
to 207Pb and 206Pb) because initial compositions
are constrained to lie along lines whose slopes are controlled only by age (i.e. isochrons). For the Abas terrane, Whitehouse et al. (1998) have re-ported c 760 Ma UPb zircon ages from two
typical granitic gneisses. Projecting the Abas com-positions back along 760 Ma isochrons indicates a slightly larger range of207
Pb/204
Pb ratios for com-parable 206
Pb/204
Pb ratios to the Afif terrane of Saudi Arabia (:800 – 600 Ma Stacey and Kramers (1975) model206
Pb/204
Pb; Group III and filled symbols in Fig. 3a), most likely due to a greater range of pre-760 Ma U/Pb ratios and/or different protolith ages. In addition, there are no 760 Ma projected initial compositions or feldspar analyses from the Abas samples of the Grolier et al. (1977) collection that are less radiogenic than Afif Group III, indicating the dominant continen-tal character of these gneisses compared with the juvenile arc terranes of the Saudi Arabian shield (Groups I and II, Fig. 3a). A similar inference may be made about the Al-Mahfid granitic gneisses which have been dated at c 760 Ma (Whitehouse et al., 1998; Fig. 2a), although it is clear from the inset diagram that the highly radio-genic whole-rock compositions of many of the Al-Mahfid samples project to much higher 207Pb
/
204Pb ratios for 600 – 800 Ma Stacey and Kramers
(1975) model 206Pb/204Pb ratios than either the
Abas or Afif samples. A late-Archean igneous crystallisation age with a later episode of Pan-African Pb-loss has been described in the zircons from two of the Al-Mahfid granitic gneisses (Whitehouse et al., 1998) and the highly radio-genic compositions require high U/Pb ratios both before and after Pan-African age event(s). The Al-Bayda granitoid lithologies show similar be-haviour to the Abas granitic gneisses, although the Al-Bayda gabbros plot close to the Group I and II arc rocks of Saudi Arabia. An unusual feature of the feldspar Pb-isotope data from the Al Bayda arc terrane is that these display highly unradiogenic compositions, plotting below the Stacey and Kramers (1975) terrestrial Pb isotope growth curve and to the left of the fields defined analyses, these have been kindly made available to
us by J.E. Baker). These are shown in relation to the fields for feldspar Pb-isotopic compositions of Pan-African rocks in Saudi Arabia — groups I and II represent arc rocks, respectively, west and east of the Nabitah suture zone (Stoeser and Stacey, 1988) and Group III represents 750 Ma and younger intrusives in the Afif terrane (Agar et al., 1992); pre-750 Ma (pre-Siham arc) granitoids and metasediments (Agar et al., 1992) are shown as individual data points. The feldspar Pb-isotopic data from Saudi Arabia are considered by Agar et al. (1992) to reflect compositions in the intervalc
M.J.Whitehouse et al./Precambrian Research105 (2001) 357 – 369 364
by feldspars from arc rocks of Saudi Arabia (Groups I and II, Fig. 3a). Furthermore, as dis-cussed above, these rocks have unusually high SmNd model ages for juvenile Pan-African arc
rocks. These features of the data may be ex-plained if juvenile Al Bayda arc rocks (? c 750 Ma, Whitehouse et al., 1998) assimilated substan-tial amounts of ancient crustal material or if the arc itself contains accreted fragments of older
continental material which should be tested by further geochronological studies. The four sam-ples from western Yemen project back into the Group II Pb field of Saudi Arabia, suggesting closer affinity with the arc terranes of Saudi Ara-bia than with either the continental Afif terrane, or the Abas and Al-Mahfid terranes of Yemen.
Interpretation of whole-rock data for the com-bined uranogenic – thorogenic Pb-isotope
M.J.Whitehouse et al./Precambrian Research105 (2001) 357 – 369 365
Fig. 4. Correlation plots of uranogenic and thorogenic Pb-iso-tope ratios (206Pb/204Pb and 208Pb/204Pb, respectively) with Nd-isotope ratios (143Nd/144Nd); whole-rock data only.
terranes may be expected to influence the compo-sition of any recent volcanics erupted through the crust. In Fig. 4, we show whole-rock 208Pb/204Pb
and 206Pb
/204Pb ratios (Table 2) plotted against
whole-rock 143Nd/144Nd (Table 2 and Windley et
al., 1996). A number of observations may be made using these isotope correlation diagrams: (1) only the Al-Mahfid granitic gneisses extend to
143Nd/144Nd ratios Bc 0.5115 (
oNd(0))B−22);
(2) while there is clear overlap in Nd-isotopic compositions \0.5115 between Abas granitoids and gneisses, Mahfid granitic gneisses and Al-Bayda plutonic rocks, the Abas compositions are in general characterised by low and restricted Pb-isotopic signatures; (3) basement rocks of western Yemen have PbNd signatures that are
highly distinctive from those of the Abas, Al-Bayda and Al-Mahfid terranes.
3. Correlations with neighbouring regions
Whitehouse et al. (1998) discuss possible ter-rane correlations between the Precambrian of Ye-men and neighbouring regions of Saudi Arabia and Somalia. For the purposes of this review, we reproduce and embellish this discussion here, to-gether with new insights provided by the Pb-iso-tope data. Table 1 (modified after Whitehouse et al., 1998) summarises available geological, iso-topic and geochronological data.
3.1. Saudi Arabia
Windley et al. (1996) correlated terranes in northwest Yemen and Saudi Arabia on the basis of descriptions of the observed geology together with reasonable geometric constraints since there are, to date, no geochronologic and/or isotopic data from northwest Yemen. The two Precam-brian basement terranes we recognise in northwest Yemen may be correlated with the Asir and/or Afif terranes of Saudi Arabia (Stoeser and Camp, 1985; Johnson, 1998) by direct extrapolation of the boundary between them represented by the 680 – 640-Ma Nabitah orogenic belt (Quick, 1991; atics shown in Fig. 3b is more complicated
be-cause initial compositions will lie along a pro-jected slope determined by age and Th/U ratio. Thus, although it is reasonable to assume ac760 Ma age for projection, little may assumed about whole rock Th/U ratios for these rocks (and in fact whole-rock Th/U ratios derived from mea-sured Th and U concentrations would likely be in error because of the mobility of U in near-surface weathering environments). Qualitatively, however, it is clear that some of the Al-Mahfid samples exhibit much lower post-750 Ma Th/U than the Abas samples since they show a similar range of
208Pb
/204Pb ratios for much greater corresponding
values of206Pb/204Pb (Fig. 3b, inset).
2.2.3. Whole-rock PbNd isotope correlations
M.J.Whitehouse et al./Precambrian Research105 (2001) 357 – 369 366
Fig. 1). This correlation assumes that there are no hidden sinistral strike-slip faults belonging to the Najd system between c 18°N where the Nabitah suture is obscured by the Wadi Tarib batholith and the Yemen – Saudi Arabia border atc17.5°N (see also discussion by Johnson and Stewart, 1996 and Whitehouse et al., 1996). North-northeast of Sada, the inferred continuation of the Nabitah belt is marked by a 40 km long series of ophiolite complexes together with island-arc type andesites, rhyolites, pillow basalts, and silicic to intermedi-ate tuffs (Michel et al., 1989). The belt extends south-southwest to Hajjah where an extensive ophiolite (Windley et al., 1996), comprising de-formed serpentinites, gabbros and deformed sheeted basic dykes, is cut by undeformed basic dykes. The suture zone is stitched by post-tectonic granites.
To the west of the Nabitah orogenic belt, the Asir terrane consists of alternating belts of green-schist-grade volcanic and sedimentary rocks and high-grade gneisses. In southern Saudi Arabia, the eastern part of the Asir terrane occurring within the Nabitah orogenic belt has undergone exten-sive plutonism and high-grade metamorphism.
The terrane to the east of the Nabitah orogenic belt in Yemen, which we tentatively correlate with the Afif terrane of Saudi Arabia (cf. Johnson, 1998, who correlates it with the Asir), comprises monotonous orthogneisses of unknown age inter-calated with arc-type pillow basalts, andesites and rhyolites, the gneisses and lavas being intruded by undated post-tectonic, granitic to gabbroic plu-tons (Michel et al., 1989). The arc has a continen-tal basement and is tentatively interpreted as a Pan-African Andean-type continental margin.
At present, there are no isotopic data available from the Asir and Afif terranes as we interpret them in Yemen. If our correlation is reasonable then the Pb-isotopic compositions for the western (Asir) terrane should overlap with Group I of Saudi Arabia, with more radiogenic compositions (Groups II and III) for the eastern (?Afif) terrane. The occurrence of rocks with Group II Pb-iso-topic signature c 70 km south of Hajjah (asterix symbol in Fig. 1) suggests that the hidden south-ward extension of the Nabitah suture passes to the west of this point.
3.2. Somalia
In Table 1, we summarise available geological and isotopic data from terranes in Yemen and Somalia, although the type of isotopic data is not readily comparable. There are no Nd model ages (tDM) from terranes in Somalia to compare with
the Yemen data presented by Windley et al. (1996) and conversely, the large amount of UPb
geochronology available for northern Somalia (Kro¨ner and Sassi, 1996) can be compared with only a handful of UPb zircon dates currently
available from the Al-Mahfid terrane of Yemen (Whitehouse et al., 1998). As a result, most of the correlations are based upon geological and geo-metric (Gulf of Aden fracture zone patterns) con-straints. Where geochronological data can be compared, this is done to emphasise the probable validity of the correlation.
Four terranes in Yemen correlate well geologi-cally with four similar terranes in northern Soma-lia (Sassi et al., 1993; Lenoir et al. 1994; Kro¨ner and Sassi, 1996) as follows (Table 1): The Al-Bayda and Al-Mukalla arcs are equivalent, re-spectively, to the juvenile volcanic-dominated Abdulkadir and Maydh (Mait) terranes in Soma-lia. The terranes in Somalia, however, are much narrower than those in Yemen suggesting south-ward discontinuation or destruction of the arcs between the bordering accreted gneiss terranes. Thus, further south in Somalia only gneissic ter-ranes are present, and we suggest that this is the manner in which the arc-gneiss terranes of Yemen pass along strike into the gneissic Mozambique belt of Ethiopia and East Africa. The intervening Al-Mahfid gneissic terrane is similar in lithologies, long crustal history and predominant east-west strike to the Qabri Bahar – Mora terrane (com-plex) in Somalia.
M.J.Whitehouse et al./Precambrian Research105 (2001) 357 – 369 367
dolomites, calcareous sandstones, quartzites, silt-stones and shales. Beydoun (1966) suggested the Ghabar Group is equivalent to the Inda Ad Group of northern Somalia. The Inda Ad Group consists of low to very low grade clastic sedimen-tary rocks and marbles (Sassi et al., 1993; Kro¨ner and Sassi, 1996). Abbate et al. (1981) suggested that the elastic rocks were derived by erosion of a volcanic arc. Both the Ghabar and Inda Ad Groups are intruded by post-tectonic granites, and both have been assigned to the uppermost Proterozoic to Lower Cambrian.
About 250 km east of Al-Mukalla town, there are the remains of another possible island arc which, considering the distance, is probably dis-tinct from the Al-Mukalla arc, although the inter-vening ground is covered by younger sedimentary rocks. This can be termed the Tha’lab arc after the Tha’lab Group of Beydoun (1966).
The late-Archean crust forming, part of the Al-Mahfid terrane of Yemen (tDMmodel ages ofc
3.0 – 2.7 Ga, Windley et al., 1996, confirmed by UPb SHRIMP analyses, Whitehouse et al., 1998)
has not to date been recorded in the Qabri Bahar-Mora complexes (terrane) in northern Somalia. The 1820 – 1400-Ma zircon xenocrystic ages from granitoids within these terranes in Somalia remain undetected in the Al-Mahfid terrane although, like the absence of late-Archean crust in Somalia, this might represent an artefact of the presently limited sampling in both terranes.
Evidence of the earliest Pan-African event of Somalia (840 Ma, Kro¨ner and Sassi, 1996) has not been found in the Al-Mahfid terrane of Ye-men. In the latter, the earliest Pan-African age so far recorded is 760 Ma in intrusive granitoids (Whitehouse et al., 1998), although involvement of older crust is indicated by c 1.3 – 2.2 Ga tDM
model ages (Windley et al., 1996).
A 814 – 778 Ma gabbro-syenite belt is promi-nent in the Qabri Bahar – Mora complexes (ter-rane). Comparable gabbro bodies up to 6 km across occur in the Al-Mahfid terrane and some are discordant to the host gneisses (Isakin and Degtyariov, 1990), like those in Somalia, but they have not yet been isotopically dated. We agree with Kro¨ner and Sassi (1996) that these gabbros could be related to subduction – accretion
pro-cesses further north. This event may relate to an early stage of the collision of the Abas and Al-Mahfid terranes which involved accretion of the intervening 760 Ma Al-Bayda oceanic arc. South-eastward dipping subduction responsible for the accretion of this arc might have emplaced (sub-duction-derived?) gabbro-syenite complexes into the deep crust of the gneissic terrane to the east and heat input from the underlying subduction zone could have caused partial melting of that gneissic terrane with resultant formation of crustal melt granites. Zircons from a granodioritic gneiss in the Qabri Bahar complex have a 207
Pb/
206
Pb age of c 760 Ma (Kro¨ner and Sassi, 1996), which these authors associated with migmatisa-tion and anatexis caused by heating of the crust by the gabbro-syenite bodies. This event could be related to the emplacement of the crustal-melt dominated granitoid sheets at 760 Ma in the Al-Mahfid terrane.
4. Summary
M.J.Whitehouse et al./Precambrian Research105 (2001) 357 – 369 368
geochemical and isotopic signatures of Cenozoic volcanism must be taken into account in any consideration of magmatic petrogenesis in this region.
Acknowledgements
Constructive reviews by Matthew Thirwall and Joel Baker are gratefully acknowledged. We also thank Joel Baker for permission to use his unpub-lished isotopic data from western Yemen. Roy Goodwin provided invaluable technical assistance for the isotopic work undertaken in Oxford. M.J.W. acknowledges support of a Harkness Fel-lowship (Menlo Park) and the Royal Society of London (Oxford).
Appendix A. Yemen MJG-series
A number of samples analysed for this study were collected during reconnaissance fieldwork in Yemen undertaken in the 1970s by the US Geo-logical Survey in cooperation with government agencies of the former Yemen Arab Republic (i.e. North Yemen). Samples were collected along a road traverse from Dhamar to the former border between the YAR and the Peoples’ Democratic Republic of Yemen close to Mukeiras (i.e. from what we now recognise as the Abas and Al Bayda terranes). Only sample powders, feldspar sepa-rates and a few thin sections remain from this collection. Brief lithological descriptions are avail-able in Grolier et al. (1977).
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